It is known in the prior art to control the movement of a train vehicle passing through a fixed block track circuit signalling system including specific signal blocks which are established by predetermined low impedance electrical signal boundaries at the ends of each signal block. When a train is present in a given signal block at least one vehicle axle and associated wheels of the train electrically shorts between the two conductive track rails on which the wheels run. A signal transmitter operates at one end of each signal block and a cooperative signal receiver is coupled to the track at the opposite end of that signal block for providing desired occupancy sensing and control of the train vehicle movement within that signal block. A train vehicle control signal system of this general type is described in U.S. Pat. No. 27,472 of G. M. Thorne-Booth, U.S. Pat. Nos. 3,593,022 and 3,746,857 both of R. C. Hoyler et al.
The low impedance shunt boundary connections 14, 18 and 20 do not provide the desired isolation required for track signalling circuits and therefore the well known problems of pre-detection, post-detection and signal leakage are presented. In addition, for track circuits without insulated joints the injected track signals propagate in both directions in relation to the shunt boundary member 14. Many of these problems can be minimized by normalizing the signal currents within the respective signal blocks N and N+1 such that the signal level within each of the signal blocks N and N+1 is above a predetermined signal level and substantially equal.
It is known in the prior art to inject signal currents into track rails by direct injection of a voltage directly across the track rails, by inductive injection through transformer action of signal currents utilizing the low impedance shunt boundary member 14, and by inductive injection into the track rail itself through operation of a loop antenna. It is difficult to balance the signal levels in respectively adjacent signal blocks in relation to both direct injection of signal voltages directly across the track rails and inductive injection through transformer action of signal currents in relation to the low impedance shunt member 14. This is particularly true for signal blocks having different impedance characteristics such as would be provided by different block lengths. If the signal block N is approximately 150 meters or 500 feet long and the signal block N+1 is approximately 300 meters or 1,000 feet long, it is likely that the signal block N would have twice the signal current level as compared to the signal block N+1 solely because of the difference in the respective lengths and related impedance characteristics of the signal blocks. The inductive loop injection approach for the introduction of signal current into a track rail through operation of a loop antenna is operative such that by shifting the position of the loop antenna, the signal current level within the respective signal blocks N and N+1 can be balanced or normalized in relation to the signal voltage sources induced in the track rails. However, the inductive signal current injection by operation of a loop antenna has two disadvantages which cause concern in a high performance and failsafe train control system, (1) the antenna loop, because of its characteristic magnetic field, can have a signal cross-talk problem in relation to the induction of undesired signal currents in adjacent track rails, and (2) the antenna loop, because of the voltage induced in the track rails, has a reflection problem in relation to a long and short track circuit configuration. The signal cross-talk problem can be improved by using a flat plate antenna replacement for the loop antenna arrangement, as the magnetic field for a flat plate antenna is oriented differently, however, the signal reflection problem is not improved in this manner.